The present invention relates to flame-retardant Lyocell fibers which include inorganic additives incorporated therein and to their use in flame barriers, for example, in mattresses, upholstered furniture and other articles of manufacture.
Flame-retardant fibers are useful in the preparation of numerous articles of manufacture, for example, mattresses, upholstery, cars, airplanes, clothing, carpeting, etc. There is a particular need for flame-retardant fibers for use in flame barriers in mattresses and upholstered furniture.
A high number of deaths and serious injuries caused by burning mattresses were the reason for the Californian government in 2001 to begin development of a standard for the flame retardancy of these products. In 2005 the Californian standard TB 603 came into force and was adopted in the whole U.S.A. in slightly amended form under 16 CFR 1633 “Standard for the flammability of mattress sets”. According to this standard the mattress maintains flame and heat resistant integrity when subjected to gas flames (two propane-fed burners—one from the side, one from above for a period of 30 minutes) to simulate for burning bedding. The criteria for passing the test are as follows:
1. The peak rate of heat release during the whole test must not exceed 200 kW.
2. The total heat release in the first 10 minutes of the test must not exceed 15 MJ.
Mattresses which are tested according to 16 CFR 1633 usually have a multilayer construction, wherein at least one of the layers is a flame barrier. This flame barrier can be a woven or nonwoven fabric, which may be impregnated with a flame-retardant compound in aqueous solution, e.g. ammonium phosphate. This kind of flame-retardant treatment has the disadvantage that under the influence of humidity the flame-retardant compound may migrate out of the flame barrier.
Further the flame barrier may consist of inherently flame-retardant fibers like e.g. glass fibers, polyaramide, polybenzimidazole or melamine fibers.
A third type of flame barrier consists of fibers which are made flame-retardant by incorporating into the bulk of the fiber a flame-retardant additive.
All three approaches for a flame barrier may be combined in the form of fiber blends as well as by applying a final flame-retardant finish.
The overview by Horrocks, A. R. and Kandola, B. K.: “Flame retardant cellulosic textiles” in “Spec. Publ.—R. Soc. Chem. Band 224 (1998) pp. 343-362” describes the numerous approaches to make cellulosic fibers flame-retardant. The most common flame-retardant compounds for cellulose are organic or inorganic phosphorous compounds, whereby these phosphorous compounds are either applied as a finish treatment (the so-called “topical treatment”) of the fabric, which is especially used for cotton, or by using cellulosic fibers wherein a flame-retardant organic phosphorous compound is incorporated during the spinning process. The process of incorporation of a flame-retardant pigment during the spinning process is described e.g. in DE 2622569 or EP 836634. Due to the higher price of the flame-retardant organic phosphorous compound such cellulosic fibers are used preferably in textile materials which have to pass the vertical flame test according to ISO 15025. For lower flame retardancy requirements mainly inorganic additives will be used.
As a cheaper alternative to the phosphorus containing fibers flame-retardant fibers containing silica are described e.g. in EP 619848 or EP 1918431. But these fibers can only be produced by the viscose process and the yield of the silica in the final fiber is very small compared to the amount of sodium silicate used.
U.S. Pat. No. 6,893,492 discloses cellulosic fibers containing montmorillonite. These fibers show improved thermal properties compared to non-incorporated fibers, expressed as a higher residue (char) in the thermogravimetric analysis.
WO 2007/022552 discloses cellulosic fibers with incorporated unmodified hectorite for the use in products which should pass the Californian standard TB 604.
Numerous patent publications describe the use of cellulosic fibers or flame-retardant cellulosic fibers as flame barriers or as elements in mattress constructions: For example U.S. Pat. No. 7,150,059 claims the use of cellulosic fibers and especially silica-incorporated fibers for flame barriers in products which shall pass the TB 603 test. EP 1649095 claims a cellulosic nonwoven material for the use as flame barrier in mattresses which retains at least 10 percent of its fiber weight after a defined heat treatment.
As described above, silica-containing cellulosic fibers, which are made by incorporation of sodium silicate into the viscose before spinning, show a low silica yield. They can be produced only by the viscose process and because an acid process stage is needed for the formation of silicic acid from the incorporated sodium silicate they can for example not be produced by the ecologically friendly Lyocell process. Additionally the flame-retardant effect of silicic acid is low and a high percentage of silicic acid in the fibers is necessary which leads to very low mechanical properties considering the already low tenacity of the base viscose fibers. For example with a silicic acid loading of 30% to 33% in fiber the tenacity is only 12 to 15 cN/tex.
To be suitable to be incorporated in a reliable commercial scale spinning process the mentioned alternatives of flame-retardant additives montmorillonite and hectorite have to be of very high quality. This results in high production costs which are unacceptable for products in typical markets for TB 603 products.
Besides the inorganic substances mentioned above there are numerous other inorganic compounds which may be added to fibrous and/or cellulosic materials for specific purposes.
Kaolin is a crystalline clay mineral with a two dimensional sheet structure composed of units of one layer of silica tetrahedrons and one layer of alumina octahedrons. In contrast to this clays like montmorillonite or hectorite have three-dimensional structures.
Kaolin is extensively used in many industrial applications as e.g. plastics, paper, ceramics, rubber and paint. Kaolin as a filler for synthetic polymers is described in detail in the book “Functional fillers for plastics”, chapter 13; Ed. Marino Xanthos, Verlag Wiley VCH. Most of the kaolin is used in the paper industry as a coating and filler material. It is also disclosed among others for flame-retardant topical treatments of cellulosic materials in GB338654 and as a flame-retardant coating in US 2004/0226100. It is also known from DE845230 that kaolin in an amount of up to 5-10% (w/w) can be used as a matting agent for viscose fibers. DE10115941 describes the use of up to 10% of mineral additives in fibers, among others kaolin, in viscose fiber. The content of matting agent in the examples of the DE10115941 is 2% (w/w). In this low amount in the fiber of 2% up to 5-10% (w/w) kaolin will not show a considerable flame-retardant effect.
In EP 1798318 kaolin is disclosed among other inorganic compounds as a component of a halogen (chlorine) containing synthetic fiber composite for use in upholstered furniture. However there is increased reluctance to use a fiber in household products which emits hydrogen chloride when ignited.
Therefore there was a need for flame barrier materials which fulfill the requirements of standard 16 CFR 1633 and TB 603 as well as exhibiting sufficient mechanical properties and which can be produced without ecological and economical disadvantages.